1. Field of the Invention
This invention relates generally to gain control circuits, and more particularly is directed to AGC circuits (automatic gain control circuits) having utility in compressors or expanders of noise reduction circuits.
2. Description of the Prior Art
Gain control circuits are known in which the gain of a variable gain amplifier is automatically controlled by a gain control signal so as to decrease or increase the gain upon an increase or decrease, respectively, in the level of an input signal applied to the amplifier. In existing automatic gain control circuits, as aforesaid, a detector senses the output signal from the variable gain amplifier and provides a detector output coupled, through a two-stage RC circuit, to a control terminal of the amplifier. The two-stage RC circuit includes a first resistor and a first capacitor connected, in parallel between the detector output and ground, a diode and a second resistor connected, in parallel, between the output side of the first capacitor and the control terminal of the variable gain amplifier, and a second capacitor connected between such control terminal and ground. In the operation of the foregoing gain control circuit, in the case of its recovery mode, for example, when the input signal to the variable gain amplifier changes suddently from a high level to a low level, the first and second capacitors, which have been previously changed by the relatively high output of the detector, are discharged in sequence through the first and second resistors in response to the reduction of the detector output. More particularly, when the level of the input signal to the variable gain amplifier suddenly decreases, the control signal for varying the gain of the amplifier is changed only gently for a period of time and, thereafter, the control signal is quickly reduced to increase the gain of the amplifier and quickly restore the level of the output signal therefrom. In the attack mode of the described known gain control circuits, that is, when the level of the input signal to the variable gain amplifier is quickly increased, it is intended that the diode is made conductive by the increase of the detector output so that the second capacitor is quickly charged to similarly increase the control signal and thereby achieve a fast attack time.
However, such fast attack time is achieved, with the prior art gain control circuit, only when the input signal is increased suddenly from a substantial initial level. In other words, if there is a rapid increase in the input signal from a low level, then it is likely that the output voltage from the detector will be too low to make the diode conductive and, in such case, the second capacitor will be charged relatively slowly through the second resistor. Therefore, with the described prior art gain control circuit, in the case of a relatively low level input signal, the attack time may be as slow as several hundred msec. Such a slow attack time will result in overshoots in the output signal from the variable gain amplifier and limitations are thereby imposed on the applications of the described prior art gain control circuit.